1. Field of the Invention
The present invention relates to welding of thermoplastic articles; and more particularly, to a thermoplastic article comprising a frictionally welded butt joint having improved strength and a method for forming the welded joint.
2. Description of the Prior Art
Frictional welding of thermoplastic components is well established in the art. Frictional welding includes the techniques of linear vibration welding, orbital welding and spin welding. In each of these techniques, the process is accomplished by placing the two workpieces to be welded in stacked, juxtaposed relation, applying a compressive force between the workpieces and then applying a vibrational, orbital or rotational motion of the workpieces relative to one another in the plane of the interface between the two. Frictional heating of the interface causes melt down and flow of the thermoplastic material in a melt zone. Upon cessation of motion, and subsequent cooling under pressure, solidification of the material in the melt zone forms a welded joint between the workpieces.
Parts to be welded frequently are of different thicknesses. Typically, one part may be 2 to 4 mm thick and the other part 4 to 6 mm thick in the region of the weld area.
The phenomenology of the vibration welding process has been described and analyzed. See V. K. Stokes, xe2x80x9cVibration Welding of Thermoplastics, Part I: Phenomenology of the Welding Processxe2x80x9d, Polymer Engineering and Science, 28, 718 (1988); xe2x80x9cVibration Welding of Thermoplastics, Part II, Analysis of the Welding Processxe2x80x9d, Polymer Engineering and Science, 28, 728 (1988). Stokes described the welding process as occurring in four phases:
1) Heating of the interface by friction;
2) Melting and flow outward in a direction lateral to the vibratory motion;
3) A steady state at which the melting rate of the solid equals the outflow of the molten material; and
4) Solidification of the molten material when the vibratory motion is stopped.
The molten material squeezed out of the joint during the welding operation is variously called xe2x80x9cflashxe2x80x9d or xe2x80x9cflushxe2x80x9d. If the appearance of the flash is objectionable in the finished part, a separate operation may remove the flash after welding. Alternatively, the parts to be welded may incorporate xe2x80x9cflash trapsxe2x80x9d which hide the flash from view.
The strength of the frictionally welded zone is a complex function of a number of parameters. Among these are the vibrational frequency, the amplitude and direction of the vibratory motion (longitudinal, lateral, angular, orbital), the pressure normal to the interface between the workpieces, the weld time or the weld penetration (melt down) and the hold or cooling time. The effects of some of these parameters on the strengths of several unfilled thermoplastics has been reported by V. K. Stokes in xe2x80x9cVibrational Welding of Thermoplastics, Part IV: Strengths of Poly(Butylene Terepthalate), Polyetherimide and Modified Polyphenylene Oxide Butt Weldsxe2x80x9d Polymer Engineering and Science, 28, 998 (1988).
For many applications, such as automotive under-the hood applications, power tools and others, it is necessary to incorporate reinforcing fibers in the base thermoplastic materials. These reinforcing fibers, such as glass, carbon, metal, aramid or other fibers, greatly increase the strength, stiffness and heat distortion temperature of the base resins. The presence of these reinforcing fibers affects and complicates the relationships between the welding processing parameters and the strengths of the welds in the thermoplastic materials to be joined. V. Kagan et. al. described the vibration welding of such filled thermoplastics in xe2x80x9cThe Optimized Performance of Linear Vibration Welded Nylon 6 and Nylon 66 Butt Jointsxe2x80x9d, Plastics-Racing into the Future, Proceedings of the SPE 54th Annual Technical Conference and Exhibits, p.1266-1274, 1996 and also in U.S. Pat. No. 5,874,146, which publications are herein incorporated by reference thereto. It was found that under optimized welding processing conditions such that fibers from one of the workpieces penetrated both into the weld, and into the other workpiece, the welds reached a maximum tensile strength. Under less than optimal processing conditions, the reinforcing fibers failed to bridge the weld region, and consequently the strengths of the welds were lower.
In each of the above studies, the workpieces to be welded had strictly planar opposing surfaces. No suggestion was made that other than planar initial interfacial geometries could be of advantage. Indeed, in xe2x80x9cVibration Welding of Thermoplastics, Part I: Phenomenology of the Welding Processxe2x80x9d, Polymer Engineering and Science, 28, 718 (1988) at P. 718, first column, second paragraph, the author states, xe2x80x9cThe vibration welding process is ideally suited to the welding of thermoplastic parts along relatively flat seams. The process can also accommodate seams whose out-of-plane curvature is small.xe2x80x9d Thus, the author indicates that non-planar longitudinal interfaces are disadvantages to be xe2x80x9caccommodatedxe2x80x9d. No comments were made about the cross-sectional geometry of the parts to be welded.
The method and articles of the present invention are to be contrasted with ultrasonic welding and ultrasonically welded articles. In ultrasonic welding, vibration is imparted in a direction normal to the weld plane rather than in the plane of the weld, commonly using an ultrasonic horn. An ultrasonic horn is a relatively low energy source. Consequently, in contrast to frictional welding, ultrasonic welding is appropriate only for relatively small parts or for spot welding.
In order that the ultrasonic energy absorbed by the workpieces is sufficient to cause local melting, it is necessary to concentrate the energy flux. This is done by use of a projection, also known as an xe2x80x9cenergy directorxe2x80x9d on the mating surface of one of the workpieces. See for example U.S. Pat. No. 4,618,516.
An energy director or projection in ultrasonic welding is a means of concentrating the energy flux. In the design of parts to be ultrasonically welded, a single longitudinal energy director (small or large) is most commonly used (See xe2x80x9cSpecification for Standardized Ultrasonic Test Specimen for Thermoplasticsxe2x80x9d, American Welding Society, AWS G1.2m/G1.2: 1999, An American National Standard, part 5, page 3). Although more than one energy director may be used under special circumstances, it is not usually done, for the reason that more than one energy director disperses the already weak energy source and makes welding more difficult and slower. An exception may be found in U.S. Pat. No. 5,540,808 where dual energy directors were used to weld a rigid material to an easily melted, flexible material. As will be seen, the geometry, purpose and function of these energy directors differ from the geometry, purpose and function of the rectangular edge projections of the present invention.
It would be desirable to provide a method of welding thermoplastic articles to obtain high strength bonds under less than optimum conditions. It would be further desirable if this method were suitable for welding rigid, fiber reinforced thermoplastics. It would be yet further desirable if the method were suitable for forming welds of substantial dimension. Especially needed are strong, frictionally welded, rigid, fiber reinforced thermoplastic articles.
The invention provides a frictionally welded, reinforced thermoplastic article having improved strength. This is accomplished by restricting lateral flow of molten material out of the gap between the workpieces sufficient to maintain a molten pool of substantial depth from the beginning of melting to the onset of solidification. The restriction to lateral flow of material out of the gap between the workpieces is provided by dams (projections) of essentially rectangular cross-section at each lateral edge of one of the workpieces, while the other workpiece has a substantially flat mating surface.
Generally stated, the invention provides a frictionally welded thermoplastic article comprising a first thermoplastic workpiece and a second thermoplastic workpiece. Each of said first and second thermoplastic workpieces have a mating surface. The mating surface of the first thermoplastic workpiece and the mating surface of the second thermoplastic workpiece are joined in a melt down region. Prior to welding, the mating surface of the first workpiece has been comprised of a restriction to lateral flow of the melt from between the workpieces. The mating surface of said second workpiece is substantially flat.
More specifically, there is provided in accordance with the invention, a vibration welded thermoplastic article, comprising: a first thermoplastic workpiece and a second thermoplastic workpiece, each of said first and thermoplastic workpieces having a mating surface; said mating surface of said first thermoplastic workpiece and said mating surface of said second thermoplastic workpiece being joined in a melt down region; said mating surface of said first workpiece having been comprised, prior to welding, of a restriction to lateral flow of the melt from between the workpieces, and said mating surface of said second workpiece being substantially flat; wherein the restriction to lateral flow of the melt from between the workpieces is a substantially rectangular projection along each lateral edge of the first workpiece; wherein the thickness of each of said projections is between about 5% and about 35% of the thickness of the first workpiece; and the height of each of said projections, relative to the lowest point on the initial mating surface is at least about 25% of the dimension of the melt down region.
In addition there is provided by the invention, a vibration welded thermoplastic article, comprising: a first thermoplastic workpiece and a second thermoplastic workpiece, each of said first and thermoplastic workpieces having a mating surface; said mating surface of said first thermoplastic workpiece and said mating surface of said second thermoplastic workpiece being joined in a melt down region; said mating surface of said first workpiece having been comprised, prior to welding, of a restriction to lateral flow of the melt from between the workpieces, and said mating surface of said second workpiece being substantially flat; wherein the restriction to lateral flow of the melt from between the workpieces is a substantially rectangular projection along each lateral edge of the first workpiece; wherein the thickness of each of said projections is between about 5% and about 35% of the thickness of the first workpiece; and prior to welding, the cross-sectional area of the space defined by a line between the upper edges of the rectangular projections and the material surfaces between them is at least about 15% of the product of the thickness of the first workpiece and the dimension of the melt down region.
The invention further provides a method for preparing frictionally welded, reinforced thermoplastic articles of improved strength by restricting the lateral flow of molten material out of the gap between them, thereby retaining a molten pool of substantial depth between them from the beginning of melting to the onset of solidification.
Frictional welding of a first thermoplastic workpiece to a second thermoplastic workpiece is accomplished by a method comprising the steps of: pressing the first and second workpieces together under a compressive clamping pressure; moving the first workpiece relative to the second workpiece in a plane parallel to their interface sufficient to frictionally heat the interface; melting the interfacial surfaces of the first and second workpieces creating a melt down region; providing a means to restricting the lateral flow of molten material out of the gap between said interfacial surfaces; and retaining a molten pool of substantial depth between the workpieces from the beginning of melting to the onset of solidification.
The articles of this invention exhibit improved utility for automotive applications such as air intake manifolds, car cross-beams, resonators, fluid reservoirs, and air filter housings. Such articles are well suited for use in many other applications such lawn and garden equipment and power tools.